Redetermination of the crystal structure of yttrium chromium tetraboride, YCrB4, from single-crystal X-ray diffraction data

The structural parameters of yttrium chromium tetraboride YCrB4 were refined based on single-crystal X-ray diffraction data. The present study successfully refined all the positional and atomic displacement parameters of the Y, Cr, and B atoms.

A covalently bonded boron network in boride compounds plays an important role for thermal conductivity and mechanical and lattice dynamical properties.The Debye temperature � D was used to characterize these physical properties.Previous studies on intermetallic boride compounds have also proposed that the bulk � D is associated with the rigidity of the boron network (Korsukova et al., 1987;Levchenko et al., 2006;Singh et al., 2010).Using the isotropic

Figure 2
Cr and Y atoms settle in the center of the pentagonal CrB 10 and heptagonal YB 14 prisms, respectively.atomic displacement parameter U iso and Debye approximation (Willis & Pryor, 1975), the � D were derived using the following equation: , where h is Planck's constant, m is the mass of the atom, and k B is the Boltzmann constant.The mean square <U iso 2 > for B atoms was calculated using the average U iso for the boron sites.The anisotropic displacement parameters (ADPs) for each atom are listed in Table 2, with no significant anisotropy being observed in the ADPs of any atom (Fig. 3).The estimated � D for Y, Cr, and B were 413 (2), 524 (3), and 996 (25) K, respectively.Candan et al. (2019) studied the lattice-dynamical properties of YCrB 4 using density functional theory and gave a calculated � D of 965 K that corresponds well with our estimated � D for the B atoms.This result indicates that the bulk � D of the AlB 2 -type-analogous compounds can be estimated from the ADPs for the B atom.

Synthesis and crystallization
The starting materials were Y (99.9%),Cr (99.95%), and B (99.5%).They were weighed in an atomic ratio Y:Cr:B = 1:1:4.The mixture was melted in an argon-arc melting furnace (ACM-01, Diavac).The product was then turned over and remelted three times to improve its chemical homogeneity.Homogeneous YCrB 4 crystals were obtained.

Refinement details
Refinement was conducted using a space group of type Pbam, as reported by Kuz'ma, 1970.A correction for isotropic extinction was applied during the least-squares refinement.
Final refinements were performed with inclusion of anisotropic ADPs to each atom.The final refinement results are listed in Table 3.The refinement was successful, with the R factor converging without any problems and no noticeable residuals.
The Y and Cr atoms lie on the Wyckoff sites 4h (x, y, 0), and the B atoms occupy the 4g (x, y, 1/2) site.The anisotropic displacement factor exponent takes the form: Table 3 Experimental details.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å
defined as a third of the trace of the orthogonalized U ij tensor.U 12 = U 23 = 0.